This invention relates to sample coolers for cooling liquid samples and keeping them cool before they are subjected to an analysis by an apparatus for automatically analyzing a liquid sample such as a liquid chromatograph. The invention relates also to methods of cooling such liquid samples and keeping them cool.
A liquid chromatograph carries out an automatic analysis by mounting vessels preliminarily sealing in small amounts of samples to a rack, setting this rack to an automatic sample injector, causing the automatic sample injector to sequentially suck up the samples from these vessels mounted to the rack and injecting them into the liquid chromatograph according to a specified program. In most situations, those of the samples on the rack waiting to be analyzed are left under the condition of a room temperature but there are also situations that some of the samples must be kept at a lower temperature condition in order to prevent decomposition or deterioration. In such a situation, a sample cooler is employed in order to keep the sample under a cooled condition.
Conventional sample coolers are either of the direct cooling type or of the air cooling type. A sample cooler of the direct cooling type uses a rack made of a metallic material with high thermal conductivity and a cooler such as a Peltier element is attached to the bottom of the rack such that the temperature of the sample can be controlled mainly by heat conduction through solid materials. With a sample cooler of the air cooling type, essential parts of the automatic sample injector including the rack are enclosed inside a heat insulating case and the air inside the case is cooled such that the sample temperature is controlled through the air.
Next, these two kinds of conventional sample coolers will be explained more in detail.
FIG. 2 shows one of conventional sample coolers of the direct cooling type. The user will initially place liquid samples 4 inside vessels 2 (usually small glass bottles) and closes each of their openings with a septums 3. (Strictly speaking, numeral 3 indicates both a cap and a septum but is herein simply referred to as the "septum".) These vessels 2 are mounted onto a rack 1 taken out of an automatic sample injector 7. The rack 1 is made of aluminum and is provided with about 100 holes 5 for accepting these sample-containing vessels 2. Heat (including cold heat) is transmitted to these vessels 2 through the bottoms, as well as the inner surfaces, of these holes 5.
After the sample-containing vessels 2 are mounted to the rack 1, the rack 1 is set on top of a metallic block 23 inside the injector 7. The metallic block 13 is adapted to be cooled by means of a Peltier element 21 attached to its bottom surface, while its upper surface makes a close contact with the bottom of the rack 1 so as to serve as an efficient heat conductor therebetween. It now goes without saying that the rack 1 itself also serves as an efficient heat conductor to the vessels 2.
Numeral 25 indicates a temperature controlling circuit. Its function is to compare a target temperature set through a target temperature setting means 26 and a signal received from a temperature sensor 24 which is buried inside the metallic block 23 and is adapted to detect its temperature and to control the electric current flowing to the Peltier element 21 such that the difference between the temperature of the metallic block 23 and the target temperature will approach zero and hence that the temperature of the liquid samples 4 will be kept at the level of the target temperature. Attached to the back surface (the heat-radiating surface) of the Peltier element 21 on the side facing the interior of an air duct 27 are heat radiating fins such that the heat transmitted from the metallic block 23 is radiated out and away through these fins and with the aid of an air current caused by a fan 28.
The rack 1, the vessels 2 and the liquid samples 4 therein are thus maintained at a specified low-temperature level. The rack 1 is covered with a heat insulating cover 6 in order to be kept at the desired low-temperature level. The top parts of the vessels 2 surrounding their septums 3, however, are exposed from this cover 6 such that samples can be extracted therethrough by means of a sampling needle 13.
The sampling needle 13 is adapted, according to a program, to move freely not only forward, backward, to the left and to the right but also upward and downward by means of a suitable mechanism (not shown), to draw a liquid sample 4 from a vessel 3 by penetrating its septum 3, to transport the drawn liquid sample 4 to the inlet 12 of the liquid chromatograph and to inject the transported liquid sample 4 into the chromatograph so as to have an analysis carried out. Since each analysis by the liquid chromatograph takes tens of minutes, some of the liquid samples 4 mounted to the rack 1 may have to wait for tens of hours before they are analyzed. Since the liquid samples 4 are maintained at a desired low-temperature level, however, decomposition and deterioration of the liquid samples can thus be avoided.
FIG. 3, in which some of like components are indicated by the same numerals as in FIG. 2, shows one of conventional sample coolers of the air cooling type. An essential portion of the automatic sample injector 7 including the rack 1 having sample-containing vessels 2 set thereon is enclosed inside a heat insulating wall 11 to form a thermostatic chamber 10. Although not shown in FIG. 3, a portion of the heat insulating wall 11 is provided with a door through which the rack 1 can be moved into and out of its interior. Unlike the rack for the direct cooling type, the rack 1 for the air cooling type is made of a porous thin plate of a metallic material in order to allow air to circulate and to reduce its heat capacity since air is the heat-carrying medium in the air cooling type. Thus, the space surrounding the vessels 2 mounted to the rack 1 and the space inside the thermostatic chamber 10 are thermally equivalent.
A Peltier element 31 is again used as a cooling device. Since this cooling device is for cooling the air inside the chamber, the metallic block 33, onto which the heat-absorbing surface of the Peltier element 31 is attached, is provided with fins on its inner surface facing the interior of the thermostatic chamber 10 so as to improve the efficiency of exchanging heat with the air inside.
A temperature controlling circuit 35, like the one described above with reference to FIG. 2, serves to compare a target temperature set through a target temperature setting means 36 and a signal received from a temperature sensor 34 which is buried inside the metallic block 33 and is adapted to detect its temperature and to control the electric current flowing to the Peltier element 31, as explained above with reference to FIG. 2. Attached to the heat-radiating surface of the Peltier element 31 on the side facing an air duct 27 are heat radiating fins 32 such that the heat transmitted from the metallic block 33 is radiated out and away through these fins and with the aid of an air current caused by a fan 38.
The cooled air circulates throughout the interior of the thermostatic chamber 10 through natural convection but a fan may be provided inside to cause forced circulation of the air.
Since water vapor in the air is condensed on the surface of the cooled metallic block 33, a draining receptacle 14 and a draining tube 15 connected thereto and leading to the exterior of the thermostatic chamber 10 are provided for discharging the condensed dew. The air inside the thermostatic chamber 10 is thus dehumidified such that the absolute humidity inside is lowered as the temperature drops. The sample-containing vessels 2 on the rack 1 are thus enveloped by cooled and dehumidified air and are maintained at a desired low-temperature level.
Of the two types of sample coolers, the direct cooling type can remove heat with a higher efficiency and the desired low-temperature level can be reached more quickly but the vapor in the atmosphere is condensed during the cooling process. At the time of the sampling, the condensed dew is attached to the tip of the sampling needle 13 and becomes mixed into the sample, tending to adversely affect the accuracy of the analysis. Moreover, the condensed dew may contaminate the vessels 2 and the rack 1 when they are handled.
As for the air cooling type, there is no problem of dew condensation on the vessels 2 or the rack 1 because the cooling is effected with dehumidified air. Since air with a small thermal capacity is used as the thermal medium to cool the entirety of a thermostatic chamber with a large thermal capacity, however, it takes a relatively long time for the cooling. The cooling can be accelerated by using a powerful cooler and providing a fan inside the thermostatic chamber for forced circulation in its interior, but the energy consumption therefor increases faster than the speed of cooling, and hence it is not economically feasible.